When tablets are pressed in rotary tablet presses, the weight of the tablets to be produced is determined by the filling volume of the powder which is to be compacted and which is introduced into the molds. It is therefore important that the molds, which are closed at the bottom by the bottom plungers, always receive the same powder volume before pressing. The bottom plungers, which are raised and lowered by a plurality of control cams during the rotation of the rotating mold table, are subjected to considerable forces of acceleration during these vertical movements. Since it has hitherto been practically impossible in terms of production technology to achieve cooperation between the plungers and the control cams which is free from play, it is necessary to use a damping element with the plungers in order to prevent an overshoot of their stroke movement, namely to prevent lift-off of the plunger from the control cam, which would otherwise result in inaccuracies in the degree of filling and thus in the weight of the tablets to be pressed.
In the past, what are termed brake pins have been employed as damping elements. These are inserted in a transverse bore leading into the plunger and are pressed against the respective plunger by a pressure spring via an adjustable pressure plate, and thus exert a force on the plunger which is oriented transversely to the direction of movement thereof, due to which the frictional force is increased between the plunger guide and the plunger which is movable therein. In a similar manner, it is also possible to use a spring pressure piece, such as those which are frequently used for snap-in locking devices and which are obtainable as standardised components, to each plunger. This spring pressure piece is then likewise disposed in a transverse bore leading into the plunger guide, and its spring-loaded pressure head presses against the plunger transversely to the direction of movement thereof, so that the plunger is subjected to damping during its vertical movement as a consequence of the frictional force which results from this.
These known solutions have the disadvantage that it is very difficult exactly to adjust the prestress of the spring and thus the transverse force which is exerted on the plunger, in order firstly reliably to prevent overshoot of the plunger during its stroke movement and secondly to keep the frictional forces between the plunger guide and the plunger shank as low as possible, so as to prevent unnecessary wear. The adjustment of the spring stress, which is usually made by means of a screw, is seldom maintained for long, as a consequence of the vibration of the machine, so that it may become necessary, even after relatively short machine running times, to readjust these mechanical damping elements. On large tablet presses in particular, which not infrequently comprise more than 50 molds and a corresponding number of bottom plungers, the adjustment of so many damping elements is very time-consuming. Moreover, these damping elements have the particular disadvantage that they generally have to be removed completely in order to dismantle the plungers, since otherwise the brake pin which presses against the plunger can engage in an undercut located on the plunger when the latter is withdrawn and thus makes complete removal impossible.
In order to reduce the time necessary for the removal, installation and adjustment of the large number of damping elements described above, it has also already been proposed that an encircling groove for the bottom plungers be provided on the periphery of the mold table in the region of the plunger guides, wherein the plungers project laterally by a slight extent beyond the base of the groove. A tensionable brake band, which can be fixed or loosened by means of a simple band clamp, is disposed in the groove, and in its installed state its inner peripheral face presses against the plungers and thus effects a damping of the movement thereof. This design does in fact have the advantage that it can rapidly be removed for the dismantling of the plungers and can be re-installed rapidly after they have been installed. However, it does not ensure that the requisite frictional or damping force, which should be as constant as possible, is exerted on each plunger.